Import 3D model from Garry's Mod (GMod)
vtx files contain textures, and i think you can easily import/export them using: http://nemesis.thewavelength.net/index.php?c=178#p178 by nem

mdl files are compiled model files (probably for performance reasons), the original source files are in smd format which you can easily import into blender using the official Valve script for the smd format:
http://developer.valvesoftware.com/wiki/Blender_Source_Tools

As for the mdl decompiling you can use Crowbar:

https://developer.valvesoftware.com/wiki/Crowbar